1. Technical Field
The present invention relates to a lubricating oil composition, specifically a lubricating oil composition having a low friction coefficient (traction coefficient) and a superior fluidity at low temperature, suitable as a transmission lubricating oil including an automatic transmission lubricating oil.
2. Background Art
In recent years, in the lubricating oil used in a transmission including a transmission for an automobile, a lower fuel consumption than ever has been required.
To improve the fuel consumption of a lubricating oil, there have been known mainly a method in which an agitation resistance is lowered by lowering a viscosity of the lubricating oil and a method in which a frictional loss is reduced by lowering a friction coefficient in a sliding part. However, there is a limit in lowering a viscosity of the lubricating oil; and therefore, in order to further improve the energy consumption, further lowering of the friction coefficient is necessary. With regard to lowering of the friction coefficient, an evaluation method based on a traction coefficient, which is a fluid friction coefficient in an elastic fluid lubrication region, is commonly used. In other words, a lubricating oil having a further lower traction coefficient is wanted.
Besides, the lubricating oil for a transmission including an automobile transmission also needs to have a superior fluidity at low temperature because it is usually used also at low temperature in a cold weather region and the like.
As to the lubricating oil as mentioned above, in the past, for example, in Patent Document 1, as the lubricating oil having a low traction coefficient, a lubricating oil composition containing as a base oil a partial ester between a polyvalent alcohol and a carboxylic acid was proposed. However, the lubricating oil like this potentially undergoes hydrolysis so that this is difficult to be used stably for a long period of time.
Alternatively, in Patent Document 2, a proposal was made as to the lubricating oil, a blend of a mineral oil with a poly(α-olefin) having a kinetic viscosity at 100° C. in the range of 15 mm2/s or more to 300 mm2/s or less, which was produced by using a metallocene catalyst. However, the lubricating oil like this was necessary further improvement in the traction coefficient and the fluidity at low temperature.
Moreover, all of the conventional lubricating oils as exemplified above use a synthetic oil having a high viscosity; and thus, these lubricating oils become expensive. Accordingly, a lubricating oil which is relatively cheap and comprises mainly a mineral oil is expected to be realized.
Under the situation as mentioned above, a lubricating oil which has a further lowered traction coefficient, a superior fluidity at low temperature, and no fear of deterioration of the hydrolysis stability, and yet comprises mainly a mineral oil, is eagerly wanted.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2010-90210    Patent Document 2: Japanese Patent Laid-Open Publication No. 2011-174000